Inverse time-overcurrent elements are a fundamental building block for electric power system protection and have been in service for many years. Inverse time-overcurrent elements are so named because of the amount of time required for the element to assert is inversely proportional to the magnitude of the current applied.
Overcurrent, undervoltage, and overvoltage elements use inverse-time characteristics to provide line, feeder, transformer, and generator protection for power system faults and for abnormal power system operating conditions. For example, relays that include these characteristics have been in service since the early twentieth century.
Older IAC electromechanical relays typically provide one of three specific inverse-time overcurrent characteristics: (1) inverse, (2) very inverse or (3) extremely inverse. FIG. 1 illustrates a prior art electromechanical overcurrent protection relay, generally designated 100, which uses an inverse time-overcurrent characteristic. As shown in FIG. 1, the electromechanical overcurrent protection relay 100 includes a number of components. For example, relay 100 may include a current tap block 102, a time dial 104, an instantaneous unit calibration plate 106, an instantaneous unit pickup adjustment 108, a target (dropped position) 110, an instantaneous unit contact 112, an instrument identification plate 114, a time-overcurrent moving contact 116, a control spring 118, a chassis contact block 120, a latch 122, a cradle 124, an induction disk 126, a damping magnet 128, an operating coil 130, a seal-in contact 132, a target (reset position) 134, a target and seal-in unit 136, target coil taps 138, a time-overcurrent stationary contact 140 and a pickup tap 142.
A user needs to select the appropriate model of the relay in order to obtain the desired inverse, very inverse or extremely inverse characteristic. These relays have two adjustable settings; the time dial (TD) and the Tap. The Tap is sometimes referred to as the Pickup. The inverse-time operating characteristic and the settings, TD and Tap, are selected at the relay setting time, and are not dynamically configurable during operation of the relay.
FIG. 2 illustrates a newer numerical relay, generally designated 200, for example, such a numerical relay is commercially available from Schweitzer Engineering Laboratories, Inc. of Pullman, Wash. under model number SEL-421. A front panel 201 includes a display 202 for showing event, metering, setting and relay self-test status information. Display 202 is controlled by a plurality of navigation pushbuttons 204. The front panel 201 also includes a plurality of programmable status and trip target light emitting diodes (LEDs) 206. A plurality of programmable pushbuttons 208 are also provided on front panel 201.
FIG. 3 illustrates a settings group tool bar menu 300 for the numerical relay 200 shown in FIG. 2. Such numerical relays 200 include an ability to select one of a number (10) of inverse-time operating characteristics. This flexibility avoids the need to specify a particular relay according to the operating characteristic requirements. In addition, there are three settings; the Pickup, the TD and torque control, similar to the electromechanical relay 100 in FIG. 1. In the example of FIG. 3, the overcurrent pickup is defined in field 303, the inverse-time overcurrent curve is defined in field 304, the inverse-time overcurrent time dial is defined in field 305 and the torque control is selected in field 307. Selection of Yes or No in field 306 determines whether the inverse-time overcurrent electromechanical reset is activated. In addition to this option, the user can select the desired operating quantity 308 from a tool bar menu 300, for example, IAL, IA1, IA2, . . . , 312L, 310L. In this example, 310L is selected in field 302. This selectivity optimizes the use of the available overcurrent elements in the numerical relay 200. The overcurrent element reset characteristic can have a fixed delay or emulate the electromechanical relay characteristic. This emulation permits proper coordination with electromechanical relays. The numerical overcurrent relay also includes a torque control equation that emulates the opening or closing of the shading coil in the electromechanical relay.
The additional options in the menu tool bar 300 are good improvements to numerical relays but the basic functionality remains the same as the electromechanical relay counter parts. These numerical inverse-time overcurrent elements have limited adaptability. In an example, the six setting groups can be selected with logic equations while the numerical relay 200 disables itself for a short period of time (longer than one cycle) during settings groups changes. During this time, the numerical relay 200 disables all relay functions including the inverse-time overcurrent element. Thus, the overall relay availability is reduced. This reduction in availability is not desirable.
Another problem with numerical protective relays with overcurrent elements (overcurrent relays) is that they are not dynamically configurable during different power system operating conditions, (e.g., a step-down power transformer is taken out of service via opening associated circuit breakers), current contributions from surrounding power system elements (e.g., feeders) may change thereby rendering the pre-selected overcurrent settings inadequate when they are utilized as part of a coordination scheme of primary and backup overcurrent relays.
For example, for a typical distribution substation with two parallel power transformers, overcurrent relays are positioned to provide protection for associated feeders as well as to provide backup power transformer protection. Further, the overcurrent relays are coordinated (e.g., a primary and backup overcurrent relay pair) such that there is minimum disruption to the power system when a fault is detected. When one of the power transformers (and its associated overcurrent relay) is taken out of service, overcurrent relays of the transformer that remains in service must be manually (or through setting group changes) re-coordinated to compensate for current contribution changes.
The Institute of Electrical and Electronics Engineers (IEEE) Standard C37.112 [1] provides an Equation (1) to emulate the dynamics of the induction disk of an older inverse-time overcurrent relay:
                                          ∫            0                          T              0                                ⁢                                    1                              t                ⁡                                  (                  I                  )                                                      *                          ⅆ              t                                      =        1                            (        1        )                                                      where            ⁢                                                  ⁢                          t              (              I              )                                =                                                    (                                                      A                                                                  M                        N                                            -                      1                                                        +                  B                                )                            *              TD              ⁢                                                          ⁢              for              ⁢                                                          ⁢              M                        >            1                          ⁢                                  ⁢                              where            ⁢                                                  ⁢            M                    =                                    I              Input                                      I              Pickup                                                          (        2        )                                          where          ⁢                                          ⁢                      t            ⁡                          (              I              )                                      =                                            (                                                t                  r                                                                      M                    2                                    -                  1                                            )                        *            TD            ⁢                                                  ⁢            for            ⁢                                                  ⁢            0                    ≤          M          ≤          1                                    (        3        )            and where:                A,B,N—are constants that define the inverse-time relay operating characteristics.        tr—is the reset time for M=0        T0—is the operating time        M—is the relay pickup multiple        IPickup—is the relay pickup current setting (threshold)        IInput—is the relay input current magnitude        TD—is the relay time dial        
FIG. 4 illustrates a diagram which utilizes Equations (1), (2), and (3) to determine the value of outputs 51T and 51R. Block 402 receives the value of M on line 404, and Block 406 receives the value of M on line 404. If M is greater than 1, switch 426 contacts node 424. If M is not greater than 1, switch 426 contacts node 422. Thus in the case that M is greater than 1, Block 412 receives the output of Block 406 on line 408. Further, in the case that M is not greater than 1, Block 412 receives the output of Block 402 on line 410. The result is that when M is greater than one, Equation (2) in Block 406 is used to determine t(I), and when M is not greater than one, Equation (3) in Block 402 is used to determine t(I). Integrator 412 is enabled to begin the integration in accordance with Equation (1). The output 414 of block 412 is received at the non-inverting input of the comparator 416, where it is compared to a reference value of 1 at the inverting input 418. If the output 414 of block 412 exceeds the reference value, comparator 416 will set the 51T output on its output terminal 420 to logical 1. The output 414 of block 412 is also received at the inverting input of the comparator 430, where it is compared to a second reference value of 0 at the non-inverting input 432. If the output 414 of block 412 does not exceed the second reference value at 432, comparator 430 will set the 51R output on its output terminal 434 to logical 1.
Traditionally, given the relay pickup current setting of the inverse-time element, the relay was able to calculate an operating time, where A, B, N, Ipickup, TD were predetermined values selected at relay setting time. The IEEE and IEC (International Electrotechnical Commission) have defined standard curves by defining the values of A, B, N. Note that once the relay is set, the element has an operating time t(I) where the only variable quantity is the magnitude of the applied current, IInput. Table 1 below shows the constants to obtain standard inverse-time characteristics:
TABLE 1CharacteristicABNModerately Inverse0.05150.11400.02Very Inverse19.61000.49102.00Extremely Inverse28.20000.12172.00
Equations (1) and (2) have been implemented in many numerical relays using the constants shown in Table 1. In accordance with the present invention, it is desirable to replace the fixed and seftable constants A, B, N, IPickup, TD and IInput with variables that are updated dynamically, based on user programmable equations.
A general object of the present invention is to therefore provide a dynamically configurable relay element for use in an electric power distribution system.
Another object of the present invention is to provide methods for dynamically configuring a relay element for use in an electric power distribution system.